Trying to install Ecobee adding my own C wire

My AC system has no C wire and already tried to add one but my handler has all cables going straight to the components no board that identify the cables. I dont know anything about AC systems so i'm not going to mess with it.
My thoughts were if i don't have a C wire that it's just constant 24V power cant i just create my own with a transformer?
I got this transformer from a friend that was trying to do the same but his brother came to visit from up north and installed his C wire and he just handed me this transformer.

Is this what i need to add the C wire or is this the wrong thing? Any help will be nice. I'm not messing with it for now because not having AC in Florida is not an option LOL

What you're attempting will not work.

In a 24 VAC (volt alternating current) circuit , R is the live conductor. C is the return path for the current back to the transformer. My terms are probably all wrong, but when I took physics 30-odd years ago, I think it would have been called the common equipment ground conductor.

So you cannot have R from one transformer and C from another.

Assuming you have a single stage AC, there should be a two-conductor cable running from the outside unit back to the furnace/air-handler. One of those conductors will connect to Y from the thermostat. The other conductor will connect to the transformer's common side. You should tap into that same conductor for your C conductor.

P.S. Also - that's a DC transformer you have .....


First of all that transformer you pictured has a 24VDC output (DIRECT CURRENT), C-Wire is a 24VAC common (ALTERNATING CURRENT), so no don't put that in or you will most likely blow something up on the AC system.

Get an HVAC guy to come and identify the C-wire on your system and get a new cable installed (or connect it if you already have the blue or black wire in the cable).

This can get expensive real fast when you don't know what you are doing!


Here is good explanation, of what is the C-Wire and you will understand why this would have been a disaster. Just glad you asked before going :slight_smile:


Oh believe me i wasn't going to mess with it specially with something that was just handed to me. It was nice that it was free but nonetheless wasn't going to try without asking.
So now knowing a bit more about this transformer, what is it good for?

BTW called 2 different AC company's in my area and neither wanted the job. One actually came clean and told me there was no money to be made on just coming over to add a cable to my system. I still have 4 cables left on the cable that comes into the thermostat that were not used and they just rolled them in a bunch. The same cables are unused in the air handler in the attic.

I will see if i can identify those cables. BTW both cables, the one from the handler to the thermostat has 5 or 6 cables in it same from the outside to the air handler.

I was in the same boat as you. Installing an ecobee and missing a power wire, because the old thermostat ran on batteries.

My solution was to not call an AC "company". My wife found an hvac guy by asking at the local hardware store. Independent contractor. He'll take the small jobs, because it's just him. And he does an excellent job, because he only gets jobs by word-of-mouth.

He ran the wire and I don't think he charged me more than $80. He's been my hvac guy ever since.

If you can find one, I highly recommend it!


Same as me i want to get rid of the nest thermostat. Problem is being in Florida at this time a year most independent HVAC guys are busy making part of the money they missed during winter. There's 1 guy a friend recommended but his weekends are busy as hell and those are my only days off. will have to miss a day of work just to get it done because he has a slower load of work during the week.

Try posting on Marketplace or Craigslist (whatever is popular in your region) and asking for HVAC guy for connecting C-Wire. I'm sure plenty of guys will be willing to come out in the evening for a small payed side job,

I had my mini split lose all it's refrigerant and all the companies I tried were to busy for this, so I posted on Marketplace and within an hour I had 5 guys willing to take on the job after work and charging almost half what the "normal" companies would have charged.


There are ways to do it using a different transformer with an isolation relay, but you'd still need to understand the current wiring.

Maybe there is a wiring schematic online if you search for the model number of the air handler?

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The Issue is the "C" wire must connect to one terminal of the Air Handler 24Vac circuit.

So even if you had a some auxiliary power at the thermostat there still must be a connection to the Air Handler "C" wire.

I hesitate to say this but...a theoretical person might say it could be done with another 24Vac transformer. But for the uninitiated this is more risk of causing problems in the Air Handler.

I suggest the same as @jwetzel1492. If you run a 3** wire cable from the thermostat and Air Handler (but don't connect it). I would guess a decent A/C guy or an electrician could properly connect the ends. This way you don't have to pay for a professional to run the cable.

** unless its a very long run I would run a 5 conductor cable as a safety for future changes. Wire is cheap, running wire is expensive.


The C-ware converters do exist.
Ask Ms. Amazon for "c wire adapter for smart thermostat" (or something similar).

However, you will need some information about your existing circuitry.

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Go to amazon and search for Venestar add-a-wire. Installs very easily. I've been using one for years with a bunch of different thermostats. There is a unit you install inside your furnace and then a diode device you have to attach between your wiring and the thermostat. I had to cut a bigger hole in the wall to stuff it through but thermostats are so large these days it was easily covered. There are a bunch of videos online showing you how to install it. Just search for "Add-a-Wire". As long as your have standard heat/AC and 4 wires already, this will work. You can also use it go from 5 wires to 6 as well, if you are adding a second stage.

But, the very low-cost solution is to "steal" your G wire. If you have forced air heat in the US, odds are your furnace turns your blower fan (controlled by your G wire) when it calls for heat. The only time the G wire gets a signal is when your AC runs. So, as long as you don't mind losing independent control of your blower fan (and who really uses that separately) you can jumper your Y wire (cool) to the G contact (for the blower) inside your furnace and use what used to be your G wire as your new C-wire and everything will work the same except for the independent blower fan. I ran in this configuration with my first smart thermostat for years without a problem.

http:// www. youtube. com/watch?v= BftoUCnKYls
(just remove the spaces)
And that solution is 100% free.

Tried to include links but the forum won't let me. Sorry.

Ecobee’s used to include a Power Extender Kit for just this reason.

I might still have some lying around and if I find them I can send you one if interested.

I looked into the ecobee power extender. According to the information on their website it will not work with a 2 wire system. It needs at least a fan control wire in addition to the 2 temperature wires.

Guys, the OP stated that he has spare wires on his cable going to the thermostat, all he needs is to get the C-Wire figured out at the HVAC side and plug it. All these solutions are nice but need to have more than 2 wires connected and know what they are to work.


I had a similar issue replacing a battery powered thermostat with an Ecobee connected to my furnace. The C wire was not connected in the furnace, and the furnace manufacturer stated the furnace's 24v transformer was not strong enough to power the Ecobee.

My solution: Ecobee suggested and I used a separate plugin 24V transformer, snaking the wire from a non visible outlet to the thermostat.

Caveat: I have no idea if this will work with an HVAC system.

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I have the part you are talking about. My problem is that my air handler doesn't have any kind of board. All wires are connected directly to the components and sort of transformers inside the handler.
When i got the ecobee i went straight to the attic where my handler is with the PEK in hand and tools thinking i was going to find a board like i have seen in most handlers but i had none. That's when i knew i was in trouble because i have never dealt with a AC unit except for changing the capacitor that in my exe's house will blow every now and then. A nice HVAC guy came twice in 1 year to change it and he felt sorry for us because the visit will cost an average of $300. He told me "Come with me let me show you something" and he pretty much showed me what to do and where to get the capacitors cheap (Ebay) he was teaching me in a way that he was not getting himself in any liability. After that i changed a few capacitors and we have probably 4 replacement capacitors for this summer lol
The company would charge $150 for the capacitor and $150 for the job. I got 5 capacitors on fleabay for $30 and never called an HVAC guy unless changing it doesn't solve the problem.

Tried this and it didn't work.